In measuring the concentration of a gas by infrared technique, the method most commonly used is a non-dispersive method, i.e. a method where the absorption signal is measured through an optical transmission band filter having a transmission bandwidth typically in the same order of magnitude as the width of the spectrum band using which the concentration of the gas is studied. The measured radiation signal is in this case the integrated value of the transmissions at the different wavelengths of the transmission band.
The absorption spectrum of a gas in a molecular state consists normally of absorption bands produced by molecular vibrations and, within them, a fine structure, i.e. absorption lines, due to rotational transitions. When measured with a sufficient resolution, the absorption spectrum band of a gas consists of a large number of very narrow absorption lines. For example, carbon dioxide has a molecular vibration absorption band having a mean wavelength of 4260 nm. A more detailed analysis shows that the region is made up of more than 80 narrow absorption lines caused by rotation. The half-intensity linewidth and a relative intensity of these lines are dependent on many factors, such as temperature, self-absorption due to the long measuring path, and collisions by other molecules present in the gas mixture. In measurement signal compensation on the first two can in general easily be taken in to account by measuring the temperature and the linearization effects due to the measurement geometry on the gas concerned. On the other hand, the change, sometimes significant, due to collisions by other gas components must be taken into account specifically in order to minimize concentration errors. Changes in the half-intensity linewidth of carbon dioxide in nitrogen mixture and oxygen mixture are described in the publication APPLIED OPTICS, Vol. 25, No. 14, pp. 2434-2439, 1986: Cousin, Le Doucen, Houdeau, Boulet, Henry—“Air broadened linewidths, intensities, and spectral line shapes for CO2 at 4.3 μm in the region of the AMTS instrument”. The half-intensity linewidth of the carbon dioxide line (ordinal number 67) of a gas mixture at normal pressure is, in an oxygen mixture, 0.055 cm−1 (0.10 nm) and, in a nitrogen mixture, 0.060 cm−1 (0.11 nm) for a concentration of 5% CO2. The portion of self-broadening by carbon dioxide in these figures is only approx. 0.003 cm−1.
Polar gases such as nitrous oxide (N2O) have a much greater effect on the half-intensity linewidth than nitrogen and oxygen, discussed in the above-mentioned publication. For this reason, for example, the measurement result of the amount of carbon dioxide in a patient's breathing gas is typically corrected, for example, by measuring the concentration of nitrous oxide, as described in U.S. Pat. No. 4,423,739 and by using this result computationally to correct the carbon dioxide concentration. It is known that the effect of oxygen on the measurement result of nitrogen can be corrected in a manner similar to that mentioned in said patent, although the error is smaller. It is also possible that a gas component in a gas mixture absorbs infrared radiation in the same wavelength band as that gas component, which is to be measured. In this case the disturbing gas component distorts the measurement in the similar way as do the collision broadening effect.
The concentration of a gas is proportional to the number of gas molecules at the measurement volume and pressure. The number of molecules participating in infrared absorption is retained more or less unchanged in a collision process if the conditions do not otherwise change. Only the distribution of energy is slightly changed, causing broadening of the absorption line. The absorbance value integrated across the absorption line is thus retained practically unchanged. However, by the infrared technique it is not possible to measure the absorbance directly; instead, transmission is measured. According to the Lambert-Beer law,T=10−awhere T is transmission and a is absorbance, applies to one wavelength. Only linearization yields an absorbance value proportional to the concentration:a=−log T=log(1/T).Especially when the measuring is carried out non-dispersively within a certain bandwidth, the total transmission signal Tm will be an integral across the spectrum range of a filter λ1-λ2:       T    m    ⁢            ∫              λ        1                    λ        2              ⁢                            F          ⁡                      (            λ            )                          ·                  T          ⁡                      (            λ            )                              ⁢              ⅆ        λ            where F(λ) is the wavelength-dependent transmission function of the filter and T(λ) is the wavelength-dependent transmission function of the gas sample. This signal is linearized experimentally, since the Lambert-Beer low no longer applies. The end result is usually different from the total absorbance A, which is in practice independent of the collision broadening and is an integral across the absorbances a(λ):   A  =            ∫              λ        1                    λ        2              ⁢                            F          ⁡                      (            λ            )                          ·                  a          ⁡                      (            λ            )                              ⁢                        ⅆ          λ                .            
In fact, the size of the error in Tm depends on the bandwidth of the filter. Measured using a very narrow transmission band, which is of the same order as the total width (e.g. in the order of 0.1-1.5 nm) of an individual absorption line in the absorption band, the need for correcting the collision broadening is very small or non-existing. On the other hand, if the transmission band of the filter is even narrower (e.g. less than approx. 0.05-0.5 nm) than the full width of a single absorption line being measured, the collision broadening causes a reduction of the signal corresponding to the concentration, since the absorption peak becomes lower. Such a case is reported in publication WO-94/24528, in which one absorption peak of oxygen is measured using a very narrowband laser diode. In this case, however, the Lambert-Beer law applies with respect to the laser wavelength, and after linearization the absorbance can be integrated as a function of the wavelength, so that the collision broadening can be compensated for.
If the transmission band of a filter extends over a plurality of absorption peaks, as is usually the case when measuring carbon dioxide, there is need for correction, since the concentration reading increases as the collision broadening increases, or as infrared absorption of a disturbing gas component increases. When measuring carbon dioxide in a patient's breathing air, in which for example a considerable portion of the gas mixture may be nitrous oxide, a concentration value measured and calculated in a conventional manner may be up to 15% too high, owing to the error caused by the collision broadening, as explained above.
It is also possible that a gas component in the gas mixture absorbs infrared radiation within the wavelength range used for the measurement of carbon dioxide. As shown in FIG. 4, anesthetic agents like for example desflurane absorb radiation in the wavelength range that is used for the measurement of CO2. As a result of this, the measured CO2 concentration is bigger than the real concentration, when there is desflurane present in the sample gas mixture.
Both N2O and an absorbing component like desflurane in the gas sample make a disturbing increase in the measured absorbance in the wavelength band used for CO2 measurement. Thus, measurement errors caused by both of these disturbing effects can be decreased by the method according to the invention.
In a typical absorption measurement, the intensity of the transmitted radiation is measured and the transmittance can be calculated using the results. The output voltage of a radiation detector is usually proportional to the intensity of radiation arriving at the detector. To obtain as accurate results as possible the voltage V of the radiation detector has to be known usually in the following cases. Voltage Vdark corresponds to the situation when there is no radiation arriving at the detector. Voltage V0 corresponds to the situation where radiation passing a sample of gas arrives at the detector, but the concentration of carbon dioxide in the sample gas is zero. Voltage Vc corresponds to the situation, where the sample gas comprises a certain concentration c of CO2. When these voltages are known, the concentration c can be calculatedc=K[log(1/T)]b  {A}where K and b are detector-specific parameters and transmission T is calculated using the measured voltages                     T        =                                                            V                c                            -                              V                dark                                                                    V                0                            -                              V                dark                                              .                                    {        B        }            
The voltage V0 may change, for example, as the radiated power of the radiation source changes, as the windows of the sample chamber get dirty or as the sensitivity of the radiation detectors changes. This voltage can be measured by inserting to the sample chamber gas, which does not contain carbon dioxide. The voltage VC is measured, when the radiation passes the gas mixture under study and when the radiation is substantially within the absorption band of the gas whose concentration is being measured. Thus, the voltage VC changes with the concentration of the gas of interest. The voltage Vdark may change due to noise in the radiation detector or in the measurement electronics. Vdark can be measured, for example, by preventing radiation emitted by the source from arriving the detector or by turning the radiation source off. When a thermopile detector is used as radiation detector, it can under certain conditions be assumed that Vdark=0.
The voltage V0 can be measured by introducing to the sample chamber gas that does not absorb radiation at the wavelength band used for the concentration measurement. This method naturally interrupts the normal operation of the gas sensor.
It is also possible, continuously and without interrupting the operation of the gas sensor, to estimate the voltage V0 by using a reference wavelength band and measuring the radiation either with a reference detector or the same detector that is used for measuring the voltage VC. The reference wavelength band is in the prior art selected so that the gas mixture under study does absorb infrared radiation in that wavelength band. Thus, it can be assumed that:V0=kVref  {C}and where the parameter k can be determined, for example, by providing a gas mixture, which does not contain the studied gas in the sample chamber. In this case the transmittance T can be calculated using                     T        =                                                            V                c                            -                              V                dark                                                                    k                ⁢                                                                   ⁢                                  V                  ref                                            -                              V                dark                                              .                                    {        D        }            
If there is only one radiation detector in the gas sensor and the reference measurement is performed by alternatively inserting a measurement filter and a reference filter to the optical path, then the voltage may be measured, for example, when changing the filters.
The CO2 concentration calculated using the above formula is not an accurate concentration, if the sample gas contains nitrous oxide or other gas components that have an effect on the measured transmittance, e.g. like collision broadening.
It is possible to correct the effect of the nitrous oxide on the carbon dioxide concentration by correcting the CO2 concentration value with a constant correction term (for example, 0.5% independently of the percentage of the CO2 in the gas mixture) or with a correction term that is proportional to the CO2 concentration (for example, 0.1 times the estimated CO2 concentration). This correction requires that the presence of N2O (nitrous oxide) in the gas mixture is known. The correction term is typically selected so that it corrects the effect of the nitrous oxide when using a certain default gas mixture, for example a mixture which contains 75% N2O, 5% CO2 and 20% O2. The correction term typically does not depend on the concentration of N2O in the gas mixture.
The problem with this correction method is that it is exact only for the N2O concentration using which it was determined. For example, in the beginning of an anesthesia (i.e. in induction) when the dosage of N2O to the patient is started, N2O absorbs rapidly to the patient and the N2O concentration in the alveolar exhalation air of the patient is smaller than the default concentration. The use of the correction term determined using the default N2O concentration may thus cause a larger error to the estimated CO2 concentration than using no correction term at all. The CO2 concentration in the alveolar exhalation air of the patient is a very useful quantity that should be measured very accurately especially during the induction. If this compensation method is used it must be switched on manually or there has to be, for example, a signal from a separate N2O detection system that triggers the use of the compensation on when the concentration of N2O in the alveolar exhalation air increases a certain predetermined value.
The effect of the N2O on the measurement of carbon dioxide concentration can also be compensated by simultaneously measuring the N2O concentration in the sample gas mixture and making an N2O-concentration-dependent compensation in real time. The magnitude of the compensation depends in this case on the CO2 and N2O concentration. Using this method it is possible to compensate the effect of N2O accurately enough at all N2O concentrations. The problem here is the need for a real-time measurement of the N2O concentration that is synchronized with the CO2 measurement. It is possible to provide one sensor which can detect both gases or a separate sensor for each gas.
U.S. Pat. No. 4,423,739 describes a system where absorption at the sample gas is measured at three different wavelength ranges: A first bandpass reference filter is used to detect absorption at a wavelength range, where neither nitrous oxide nor carbon dioxide oxide absorbs radiation. The selected wavelength range is centered at 3.69 μm, and using this reference filter it is possible to keep track, for example, of accumulation of dirt in the sample chamber windows or of the intensity of radiation. A second bandpass filter is used to determine a measured concentration of carbon dioxide; the wavelength range of this filter is centered at 4.25 μm. A third bandpass filter is used to determine the concentration of nitrous oxide; the wavelength range of this filter is centered at 3.9 μm. The corrected carbon dioxide concentration is then determined using the concentration of nitrous oxide and the measured concentration of carbon dioxide. The system presented in U.S. Pat. No. 4,423,739 is a side-stream analyzer using a single beam configuration and one detector; the filters are placed to a rotary filter wheel in front of the detector.
The method presented in U.S. Pat. No. 4,423,739 makes the sensor complicated because three measurement wavelengths are needed, and because it is necessary to know, for example, all the gas components affecting the broadening of the absorption lines and their concentrations. Furthermore, ample experimental material must be obtained for the calculation of the corrected carbon dioxide concentration, because it involves an empirically determined constant describing the collision broadening.
U.S. Pat. No. 5,900,635 describes an alternative system for taking into account the effect of nitrous oxide on the determined concentration of carbon dioxide. This system can be implemented either in a single beam or in a multi-beam configuration. Absorption related to the carbon dioxide is determined using a first detector and a bandpass filter whose wavelength range is centered at 4.25 μm. A compensation measurement is carried out by placing an additional compensation filter at the radiation path. The compensation filter is a gas container, where the gas mixture comprises carbon dioxide. The gas container is a closed container, and the size of the correction is selected by the amount of carbon dioxide in the container. The correction is thus properly selected for a certain combination of carbon dioxide and nitrous oxide.
U.S. Pat. No. 6,147,351 describes further a system, where the effect of nitrous oxide on the concentration of carbon dioxide is compensated. In this system, the absorption related to carbon dioxide is determined twice, for example, by guiding the radiation through the studied gas along two optical paths whose length is different. When the length of the optical paths are correctly chosen, the measured absorption results are linearly independent and it is possible to determine the concentration of carbon dioxide in the gas mixture and the effect of the collision broadening using certain equations. Some parameters in the equations depend on the measured absorption values and, therefore, there may be need for extensive measurements to determine the parameters before the system can be used.
As discussed above, a sensor using which it is possible to determine the concentrations of both N2O and CO2 is complex and its structure, for example the dimensions of the sample chamber, is a compromise not providing the best possible measurement accuracy for either of the gases. The use of a separate CO2 sensor and N2O sensor causes problems in synchronizing the measurement results and, of course, requires two sensor systems. The use of two separate sensors is especially difficult in mainstream gas sensors, where the sensor is attached to the breathing tube and the sensor has to be very small and lightweight.